Images of Neptune captured by the James Webb Space Telescope show details of the planet and its rings with a clarity comparable only to those taken in 1989 by NASA’s Voyager 2 space probe during a planet flyby. The observations made it possible to capture details of many storms existing in Neptune’s atmosphere, including the Great Dark Spot. Webb also photographed Triton and six of the smaller moons.
Neptune is still mysterious in various ways because it’s far from the Sun and its light is dim. It’s been decades since the Voyager 2 space probe flew by the planet and had the chance to send close-up photos. For these reasons, there was certainly a lot of curiosity to see what details the James Webb Space Telescope, launched on December 25, 2021, would be able to capture.
Compared to Jupiter and Saturn, Neptune contains greater amounts of ice which include water, ammonia, and methane. Especially methane is a problem in Neptune observations because it absorbs a lot of solar emissions making it opaque. The James Webb Space Telescope’s Near-Infrared Camera (NIRCam) instrument captured the near-infrared images, which is why it doesn’t appear blue and its large moon Triton appears much brighter.
Clouds at high altitudes are clearly visible as bright streaks and spots because they’re composed of frozen methane, which reflects sunlight before methane gas absorbs it. For this reason, those clouds and their changes are among the features of the Neptunian atmosphere best observed by other telescopes such as Hubble or the Keck in Hawaii.
A thin line of brightness, though not at the levels of frozen methane clouds, surrounds Neptune’s equator. It could be a visual signature of the global atmospheric circulation that powers the fastest winds in the solar system, which can be faster than 2,000 kilometers per hour.
Among the many storms visible on Neptune is the Great Dark Spot in the southern hemisphere. It has some similarities with the famous Great Red Spot of Jupiter but also various differences. The Neptunian anticyclone seems to dissipate and reform over time while the Jovian one has been known for centuries. However, the difficulties in conducting detailed observations of Neptune could make the Great Dark Spot invisible due to clouds in the area. In short, it’s one of the mysteries concerning this planet.
The great moon Triton is clearly visible in the image because it’s very bright. The image includes six smaller moons: Galatea, Naiad, Thalassa, Larissa, Proteus, and Despina. Triton is interesting from an astronomical point of view because its motion is retrograde and the information available on its composition shows similarities with Pluto’s. These are clues that Triton was not born as a moon but as a Kuiper Belt object that was captured by Neptune.
There are various projects for space missions aimed at closely studying Neptune and Triton but, if NASA approves one, it will still take many years for that scientific mission to begin. Meanwhile, the James Webb Space Telescope started providing new information that is being analyzed.